Spall and Shear Fractures in the Spherically Converging Shells of Iron and Steels. Measurements of Energy and Residual Strains E. A. Kozlov, S. A. Brichikov, V. G. Vildanov, D. M. Gorbachev, D. T. Yusupov FSUE “RFNC-VNIITF”, , Russia, Snezhinsk, Chelyabinsk region, P.O. Box 245; Joint Russian-American Conference on Advances in Materials Science, August 31-September 3, 2009, Prague, Czech

Introduction Systematic experimental data are important (i) to verify and certify modern kinetic strength models of shear and spall strength of materials, multi-phase equations of state describing polymorphous, electron, and phase transformations in the shock and rarefaction waves, and (ii) to estimate how the explosion-products energy transform to the shells, as well as to analyse the character of recompaction of the shell material in the process. In experiments under consideration, the steel shells cannot be taken as incompressible. Single spall or even multiple spall fractures occur in them under explosive loading. These fractures result from interaction of two groups of rarefaction waves. Interference of these rarefaction waves in the shell results in occurrence of tensile stresses. If the material of the shell cannot withstand tensile stresses having certain amplitude and duration, then the spall layer separation or spall separation takes place in the shell (on the side of its internal surface).

The first spall is formed in the shell when the converging shock wave is reflected from its internal boundary for the first time, i.e. when the shell is still at the “high”radius. Being formed, the spall layer begins to move towards the center thus spending its kinetic energy for the work of plastic deformation and heating. If thickness of the HE spherical layer used for shell loading is not great enough, then explosion products quickly get unloaded in the free scatter, and shell would stop in the course of convergence. If the scatter of explosion products from a thin HE layer is confined by a heavy casing with a small gap, which is installed above this HE layer, then one can ensure that the main part of the shell would catch up with the spall layer and can observe specifics and character of material recompaction for the shell fractured at the high radius in the process of its convergence to a smaller radius. Intense explosive loading of the shell or small spall strength of its material can lead to several subsequent spall fractures.

Comparative experiments with confined scatter of explosion products is an innovative experimental setup, since it allows (with a small gap between HE and the casing) one (i) to have in the shell, during the first reflection of the shock wave from the internal free surface, just the same spall fractures as in the first experimental setup and (ii) to further follow peculiarities in recompaction of the fractured shell material in the process of shell convergence to smaller radii. This experimental setup permits recovery of converged shells, measurements of their energy and residual strain, as well as systematic material science investigations. The purpose of this work was to obtain comparative experimental data on specifics in spall and shear fractures of shells made of iron and some steels having almost similar densities under normal conditions but different equations of state, as well as strength characteristics under low-, and high-rate deformation.

Experimental Set-Up Studied Shell Jacket II Jacket I

Experimental Set-Up Consideration was given to spherical shells (49-mm nominal external diameter, 10-mm initial thickness, 380 g mass at density of 7.85 g/cm 3 ) of unalloyed high- purity armco-Fe (215–300  m grain size), steel 30KhGSA as received and after hardening to 35–40 HR с hardness, austenitic steel 12Kh18N10T. The test spherical layer consisted of two parts connected with the help of a threaded joint. This layer was installed in turn into two sealing shells of steel 12Kh18N10T with the nominal thickness 4 and 7 mm, respectively. The size, type of HE in the spherical layer, and the system of HE initiation were identical in all explosive experiments. The only difference in these experiments was presence or absence of the external casing that confines explosion products scatter. Cast iron was used as the material of the external casing. The specially developed solid-state calorimeter was used to determine energy the explosion products imparted to the compressed assembly [2]. The compressed assemblies got into the calorimeter 25–30 sec after the explosion. Quantitative data on the experimental setup of spherical explosive experiments and their basic results are given in the Table.

E SP SUP NP Figure 1. Meridional section of the armco-Fe shell after spherical explosive compression without the external casing that confines explosion products scatter NP, SP – north and south poles of the test spherical shell; E – equator or equatorial joint of spherical layer members; SUP – upper point of the shell when cooling in the gravity field. Arrow shows the direction of gravity field acceleration. 1 – first spall; 2 – second spall; 3 – third spall; 4 – peripheral part of the shell, which remained unfractured.

с d E NP 1cm NP SP E e f NP SP E E 1 cm NP 1 cm E SP b a NP SP E Figure 2. Meridional section of shells for austenitic steel 12Kh18N10T (a, b), as-received 30KhGSA steel (c, d) and 30KhGSA HR C steel (e, f) after spherical explosive compression in loading modes I and II with unconfined (a, c, e) and confined (b, d, f) explosion products scatter. 1 – first spall; 2 – second spall; 3 – unfractured peripheral part of the shell; 4 – first spall; 5 – second spall; 6 – peripheral part of the shell with the local spall fracture 7; 8 – spall layer formed under explosive loading; 9 – a trajectory of maximum shear stress locations, along which the spall layer gets fractured; 10 – unfractured peripheral part of the shell.

The viscous character of spall fracture was observed in the shell of Armco- Fe (Fig. 1) and austenite steel 12Kh18N10T (Fig. 2). Shells of the steels fractured at high radii are well compacted into the sphere in conditions of explosive compaction with confined free scatter of explosion products in contrast to shells of as-received steel 30KhGSA and especially in the hardened state (Fig. 2). Areas with incomplete compression are revealed in polar zones of the shell made of pre-hardened steel 30KhGSA. All test shells are noted to have different character of the spall and shear fractures in polar zones and in the area of equatorial joint of members forming the spherical layer being investigated. This is associated both with the initial structure present in the material of test ingots, and with small gaps in the threaded joint, which though cause transformation of the shape and parameters of the stress pulse that approaches the internal boundary of the shell in this zone. In its turn, this causes changes of spall fracture, right up to vanishing, in the area beneath the thread.

After chemical or ion etching of the meridional section, measurements of distribution of hardness H V (r,  ) and microhardness H  (r,  ) along radius r and by polar angle  in shells of armco-Fe and steel 30KhGSA revealed occurrence of three concentrically arranged zones, and namely: – zone of high-rate deformation of ferrite in the initial  -phase; this zone is adjacent to the external boundary of the compressed shell; – zone of high-rate deformation of ferrite in the range of the reversible  - phase transformation; this zone is found in layers at the deeper radius; – zone of the recrystallized structure for the first mode of explosive loading with unconfined scatter of explosion products or zone of local melting for the second mode of explosive loading with explosion products scatter confined by the external casing; this zone is close to the center. Detail results of the metallographic, as well as SEM and TEM examinations of each iron and steel shell after their explosive loading will be presented in the follow-on works.

Conclusion Experiments on spherical explosive compression and follow-on recovery of hermetically sealed shells with the different-extent spall and shear fractures are proposed and implemented. Different character of spall and shear fractures, as well as of the material compaction of shells fractured at high radii was demonstrated by the example of shells of unalloyed high-purity iron, steel 30KhGSA as received and hardened up to 35–40 HR C, as well as austenite steel 12Kh18N10T. Spherical explosive experiments with the guaranteed recovery of loaded shells and their calorimetric measurements directly after loading with the follow-on measurement of residual strain and metallographic and electron-microscopic analysis are of interest from the standpoint of monitoring constancy of dynamic mechanical properties and characteristics, i.e. shear and spall strength of materials in case of changes in technologies of their fabrication or in the process of long- term storage after their fabrication. Similar experiments on the spherical explosive compression of even thinner shells are of interest for verification and certification of kinetic strength models and multi-phase equations of state used in modern 1D–, 2D–, and 3D– computer codes.